Donations to political parties should be capped at £5,000 a year, the Labour leader, Ed Miliband, has said as he launched fresh proposals to resolve the scandal of party political funding.

Miliband claimed he was making a move that would deprive his party of millions, but his proposals were rejected within an hour by the Conservatives, who described them as meaningless as the Labour leader was planning to leave untouched the £8m annual income the party receives from union political levy payers affiliated to the party. Nearly 3 million union members are automatically enrolled into giving £3 a year, unless they choose to opt out.

The deputy prime minister, Nick Clegg, convened all-party talks on political funding after the Sunday Times revealed that privileged access was being offered to Tory donors by Conservative headquarters. The first meetings were held last week.

Miliband told the BBC's Andrew Marr programme that the £5,000 cap would mean real pain for his party as it would curtail large discretionary donations by unions, especially at election time.

But Grant Shapps, the Conservative housing minister, dismissed the idea as "a complete wheeze, one of the most disingenuous interviews I think I've seen all year". The Conservatives pointed out that, in 2011, Labour received just £100,000 in discretionary donations, less than 1% of its total £10m funding from the unions.

Labour countered that in the last election year, as opposed to last year, the party had received more than £4m in discretionary one-off donations from the unions, as part of a total of £9m high value donations it received. Labour also claimed that, over a parliament, the unions had provided £10m in one-off donations and this would be fsacrificed under Miliband's proposals.

The committee on standards in public life estimated recently that one-off union donations to Labour averaged £2.5m a year between 2001 and 2010.

In his BBC interview, Miliband also rejected proposals for union members to opt in to a political levy, as opposed to the current system whereby a union member has to decide to contract out, an option that is not always well advertised.

There are currently 28 unions with political funds, not all affiliated to Labour, and just under 10% of their 5.5m members opt out.

Differing views on how to treat Labour income from affiliations has been a repeated cause of previous failures to reach an inter-party agreement on party political funding.

In its report in November, the committee on standards in public life proposed a switch to contracting in.

But Miliband said: "The issue really is whether it's transparent where people's money is going. That's what matters. I am in favour of transparency and that does have to be looked at." On his blog, he added: "There is the world of difference between a wealthy individual giving millions, and millions of trade union levy payers paying a small sum of money to affiliate to the Labour party."

The committee proposed a cap of £10,000 on donations, but the Tories insisted the cap should be £50,000 annually.

The committee calculated that, if nothing else changed, and a £10,000 cap on donations had existed between 2001 and 2010, the Conservatives would have lost £12.7m or 76% of its income, the Liberal Democrats £1.6m or 57% of income, and Labour £7m or 42% of its income. The Labour figure assumed there was no cap on union affiliation fees.

Miliband defended his £5,000 cap, saying: "The Tories say they want to limit donations to £50,000 a year. That is twice the average annual wage. The Tories would allow an individual to donate £250,000, or a married couple to give £500,000, over the course of a parliament. A cap set at £50,000 would be unacceptable, because it would still keep big money in politics and still leave parties open to questions about buying access."

The Liberal Democrats were warmer to the Miliband initiative, but said he should shift support for political levy payers contracting in.

Simon Hughes, the deputy leader, said: "I think the most important thing is that Ed Miliband as opposition leader recognises, as Liberal Democrats have done for a long time, that we need to get big money out of politics. We argued for a cap of £10,000, the Independent Standards Commission had argued for a cap of £10,000, Ed Miliband has said £5,000, so we're in the same ballpark. The Conservative party hasn't got there yet, the Conservative party had argued for a larger cap. I think there is a consensus in Britain that big money needs to get out of politics."